Satellite positioning systems make it possible to precisely calculate the position of a receiver, for example installed aboard a vehicle or aircraft.
Several radionavigation systems exist, such as GPS, Galileo or Glonass, the best known being GPS (the acronym standing for the expression “Global Positioning System”).
For a satellite positioning system receiver, one tries to obtain the shortest possible duration of acquisition of a radionavigation signal. Indeed, when turning on a receiver, the duration of acquisition is the duration for which the positioning information cannot yet be delivered by the receiver.
The manner of operation of the GPS system is recalled succinctly. It consists of a constellation of 28 satellites and of a terrestrial network of earth reference stations. Each satellite orbits at about 22 000 km from the earth with a period of revolution of 12 hours. Each of them emit two signals, one at 1575.452 MHz for civil applications and the other at 1227.6 MHz for reserved access applications. The signal emitted by a satellite consists of a carrier, optionally of a sub-carrier in the case of a BOC or binary offset carrier modulation, modulated by a known spreading code and optionally by unknown data. The satellites all emit on the same frequencies and the signals emitted are differentiated by their code.
These codes generally exhibit a period T, which may be short, for example 1 ms, or very long on the time scale considered, for example a week, but they can also be non-periodic, this being the case for example for encrypted signals. The codes typically consist of a large number of elementary time divisions, also called code “chips” which have a mean duration equal to Tc.
The positioning of the receiver is obtained by measuring the distance between a satellite and the receiver on the basis of the duration of propagation of the signal between this satellite and the receiver. The time offset between the signal emitted by the satellite at a known date and the signal received by the receiver at a date to be determined, corresponds to the duration of propagation sought. In the receiver, a replica of the code emitted is generated locally. The date of reception of the signal is measured by setting the signal received and the local signal in phase; the setting-in-phase criterion corresponds to maximizing the correlation function for the two signals, that is to say to searching for a peak in the results for the correlation between the signal received and the local signal, assumptions of different offset between the signal received and the local signal being considered for each correlation calculation.
The correlation calculations are performed on the basis of the real and imaginary components of the signal received, resulting from a sampling of the analog radionavigation signal performed at a frequency Fe of greater than 2/Tc, Tc being the mean duration of a code chip, according to Shannon's criterion. On output from the antenna of the receiver, the signal is, in a conventional manner, converted to intermediate frequency, filtered, sampled, and then converted to baseband by digital processing, before correlation with a local code of a satellite.
A correlation calculation is based on an assumption made about the date of reception of the signal emitted by the satellite and received at the level of the antenna of the receiver. Correlation calculations are performed for various assumptions corresponding to various reception dates spaced apart by a duration of half a code chip. For a periodic code of period T equal to 1024 chips, this makes it necessary to test up to 2×1024=2048 assumptions, i.e. consequently 2048 correlation calculations to be carried out.
The correlation calculations are performed over an integration interval whose duration Tint may be varied as a function of the a priori predicted signal-to-noise ratio.
Moreover, in this case, the calculation of a correlation between the signal received and the local signal for an assumption regarding the date of reception of the code received corresponds to Tint·Fe products between samples of the two signals and then Tint·Fe−1 sums of the results of the products. When the duration of a calculation of a correlation equals DCalcul, and if the calculations of the 2048 correlations are carried out sequentially, the total duration of the calculation of the correlations then equals 2048·DCalcul. This total duration can exceed the ten or so minutes for phase-setting the code of the signal received, that is to say for accessing and using the data produced by the satellite which emits the signal.
For an integration interval of fixed duration, a first solution for reducing the total duration of the calculation of the correlations consists in reducing the duration of a correlation calculation, for example by performing the operations (products and then sums) in parallel rather than performing them in series as described previously. In this way, the total duration of calculation of the correlations is reduced, since the operations are carried out simultaneously. For such purposes, the receivers implement several correlators in parallel.
The standpoint of this solution is adopted hereinafter.
In a certain number of situations, the reduction in the total duration of the calculation of the correlations which is obtained by the first solution presented is not sufficient, this being the case, for example, when the period T of the code is long or when the number of elementary correlations to be performed is multiplied because of a significant number of assumptions to be made about the frequency of the signal to be considered in order to compensate for the Doppler effect.
We recall that the processing of the signal received comprises two phases, an acquisition phase and a tracking phase. The aim of the acquisition phase is to synchronize a code and a carrier, that are generated in the receiver, with the code and the carrier of the signal received from the satellite. This phase is iterative so as to traverse a domain of uncertainty in terms of code and Doppler effect. The aim of the tracking phase is to maintain the best synchronization of the local code and of the phase of the local carrier with the code and the phase of the carrier of the signal received, so as to produce a measurement of the position of the code and of the phase of the carrier for the positioning calculation. This phase consists in closing the code tracking loop or DLL (“Delay Lock Loop”) and carrier phase loop or PLL (“Phase Lock Loop”).
The signal acquisition time is proportional to the uncertainty in the code, to the integration time of the correlators, and inversely proportional to the number of available correlators.
The uncertainty in the code is related to the period of the code and to the uncertainty in the signal propagation time, i.e. to the uncertainty in the position of the satellite and in that of the antenna of the receiver, as well as to the uncertainty in the clock of the receiver with respect to the system time. The latter uncertainty is predominant notably in the case of signals with non-periodic codes.
The uncertainty in the code is related to the initial conditions and does not depend on the receiver, but is related to the initial conditions, unlike the number of correlators which is a parameter directly influencing the performance of the receiver.
The integration time is inversely proportional to the signal-to-noise ratio. In the presence of jamming the acquisition time can therefore be very long, thus requiring many correlators to maintain an acquisition time that is reasonable from a user's point of view.
The receivers use multi-correlators which make it possible to test several code assumptions at the same time: a bank of correlators using the same local code sequence, but offset by a delay line, is embedded inside a channel dedicated to a satellite.
The multi-correlators also serve for fast reacquisition, which is useful in the presence of intermittent jamming or masking.
The multi-correlators are also used in the tracking phase to maintain the tracking in the presence of jamming, by non-linear filtering techniques.
On the complexity of the multi-correlators, expressed in terms of number of elementary logic operators necessary to embed them in an FPGA or an ASIC, depends the cost and the electrical consumption of the receiver and its performance in terms of acquisition time.
French patent application FR 2 898 998 proposes that the total duration of the calculation of the correlations be reduced by avoiding repeating intermediate calculations which are common from one correlation calculation to another. For such purposes, this application proposes a method for calculating correlations between a first sequence and a second sequence, said first sequence and said second sequence each having a duration DCode. The first sequence is extracted from a digital signal comprising a code, said code comprising elementary time divisions, called chips, of a duration Dchip. The chips are sampled on pulses delivered by a numerically controlled oscillator, or NCO, at the mean frequency 2/Dchip, the second sequence resulting from a sampling at a frequency Fe of an analog signal. The frequency Fe is greater than 2/Dchip. This method comprises a step of aggregating the samples of the second sequence, over consecutive integration intervals of duration equal on average to Dchip/2, starting at each pulse of the numerically controlled oscillator, so as to determine elementary aggregate results. Furthermore, in an optional manner, the elementary aggregate results may be employed to determine results of calculations of correlations between a first sequence and second sequences, the second sequences being deduced from one another by a time offset of duration Dchip/2. The document also specifies that the method can comprise, for each second sequence, a step of weighting each elementary aggregate result for the second sequence by the value of the chip of the first sequence at the start of the elementary aggregate, so as to obtain weighted aggregate results, and a step of accumulating the weighted aggregate results.
Such a method pegs the spacing between two successive correlators at Dchip/2 or Tc/2.
Thus, a spacing between two successive correlators which is pegged at Tc/2 is not very precise for the tracking phase, for which a smaller spacing between two successive correlators is required, to obtain sufficient precision of the measurements. Furthermore, the working frequency at 2Fcode remains significant.